Calendar event monograms, part 3. Remarkup for calendar event monograms.

Summary: Ref T7928, Calendar event monograms, part 3. Remarkup for calendar event monograms.

Test Plan: Create calendar event, open a maniphest task, add 'E{id}' and preview should show a hovertag for event that links to event.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7928

Differential Revision: https://secure.phabricator.com/D12580